1. Field of the Invention
The present invention relates to a charging unit and a manufacturing method thereof. Further, the present invention relates to a charging device and a transfer device utilized in a copying machine, a printer and so forth of an electrophotographic system.
Electrophotography means electrostatic copying, electrostatic recording and so forth. In the field of the electrophotography, a device for applying an electric charge to a surface of a photosensitive material is called as a charging device. Meanwhile, a device for applying a voltage to a recording medium is called as a transfer device. This transfer device applies the voltage in order to transfer a toner image, which is formed on a photosensitive material, to the recording medium (a paper, a film and so forth). The charging unit and the charging device according to the present invention are not exclusive to the field of the electrophotography, but are applicable to any cases in that the electric charge is applied to a surface to be charged. The material to be charged may be any one of a drum shape, a belt shape, a sheet shape and so forth.
2. Description of the Related Art
Both of the charging device and the transfer device comprise the charging unit, and charge the material in a same principle. Their uses are merely different. For this reason, the charging unit and the charging device are mainly described hereinafter. As to a charging method in the field of electrophotography, is widely used a contact charging system (roller, tube, blade, blush and so forth) in which ozone is hardly generated, instead of a corona discharging system accompanying generation of ozone. A roller charging system is widely used as one of the contact charging system. This system uses a charging roller comprising shaft, a conductive elastic layer provided around the shaft, and a charging layer provided around them. Although the roller charging system represents the contact charging systems, there arise problems in that it costs to manufacture the charging roller, and in that it costs to peel the conductive elastic layer from the shaft when discarding or recycling, and in that the discarded material is not good for environment. In order to solve above problems, it has recently been proposed to use a seamless charging tube.
The charging roller and the charging device using the seamless charging tube is described in Japanese Laid-Open Publication No. 5-273844, for example. FIG. 9 shows a first embodiment of the charging roller described in the above Publication, and FIG. 10 shows a second embodiment thereof.
In FIG. 9, the charging roller 30 comprises a conductive shaft 2a, a conductive foam layer 2b integrally formed on an outer surface of the shaft 2a, and a seamless charging tube 2c loosely encasing the foam layer 2b without adhering to a peripheral surface thereof. In other words, an inner diameter of the charging tube 2c is larger than an outer diameter of the conductive foam layer 2b. Both ends of the shaft 2a of the charging roller 30 are held by a bearing member, and the charging roller 30 is pressed against an outer surface of a photosensitive drum 1 by means of a spring 23 provided both ends of the shaft 2a under a predetermined pressing force (1 Kg).
The whole of the charging roller 30 including the seamless charging tube 2c is driven to rotate in association with a rotation of the photosensitive drum 1. A power supply 4 applies, to the charging roller 30, a superposed oscillating voltage (Vac+Vdc) with an alternating voltage Vac (2 kVpp, 600 Hz) and a DC voltage Vdc(xe2x88x92700 V) corresponding to a desired surface voltage. The voltage is applied via a slide electrode 24 contacted to the shaft 2a. Owing to this, the voltage is applied to the charging tube 2c via the shaft 2a and the conductive foam layer 2b. Thus, charges are interchanged at a pressure nip portion between the charging roller 30 and the photosensitive drum 1 so that the outer surface of the photosensitive drum 1 is charged up to the desired surface voltage. Incidentally, the photosensitive drum 1 and the power supply are identical with those denoted by reference numerals 1 and 4 in FIG. 1. The spring 23 and the bearing member are identical with those denoted by reference numerals 3 and 11 in FIG. 2.
A charging roller 31 shown in FIG. 10 comprises a conductive elastic cylinder 2d formed in a thick cylindrical shape, the charging tube 2c integrally covering the conductive elastic cylinder 2d, and the shaft 2a loosely inserted into a hollow of the elastic cylinder 2d. The elastic cylinder 2d loosely encases the shaft 2a, and the inner diameter of the elastic cylinder 2d is larger than the outer diameter of the shaft 2a. The charging roller 31 is supported similarly to the embodiment 1 shown in FIG. 9 so as to be pressed against the outer surface of the photosensitive drum 1. The charging tube 2c closely contacts with the outer surface of the photosensitive drum 1, by the pressing force of the spring 23, at the pressure nip portion between the charging roller 31 and the photosensitive drum 1. Meanwhile, a state that the shaft 2a closely contacts with the surface of the hollow of the elastic cylinder 2d is kept. The charging roller 31 including the elastic cylinder 2d, the charging tube 2c and the shaft 2a is driven to rotate in association with the rotation of the photosensitive drum 1. Similarly to the foregoing embodiment 1, the superposed oscillating voltage is applied. The voltage is applied to the charging tube 2c via the shaft 2a and the conductive elastic layer 2d so that the outer surface of the photosensitive drum 1 is charged up to the desired surface voltage.
The inventor of the present application has confirmed the following problems of the charging roller shown in FIG. 9 and the charging device using thereof. The first problem relates to the cost. As methods for integrally forming the foam layer 2b on the surface of the shaft 2a, two methods are known. In the first method, a foam material of conductive rubber is formed encasing the shaft 2a and then ground in a predetermined thickness. In the second method, a foam material of conductive rubber is formed in a cylindrical shape by an extruding machine such that its inner diameter is smaller than the outer diameter of the shaft 2a, and then, the air is blown into the cylindrical body to enlarge the inner diameter of the foam material rather than the outer diameter of the shaft 2a. In this state, the shaft is inserted into the cylindrical body. The cost of the second method is lower than that of the first method, however, a considerable cost is spent in both methods. The latter method needs the cost for inserting the shaft into the hollow of the cylindrical body, besides the cost for merely forming the cylindrical body.
The second problem relates to a shift of the charging tube 2c during the rotation. Since the charging tube 2c loosely encases the outer surface of the foam layer 2b, the charging tube 2c moves little by little in an axial direction in accordance with the rotation of the photosensitive drum 1. Manufacturing accuracy of the respective parts has a limit so that combination of ideal dimensions can not be obtained relative to the respective parts. Thus, in practice, it is impossible to prevent the charging tube 2c from moving in the axial direction. Owing to this movement, an end portion of the charging tube 2c abuts on the bearing member of the shaft 2a. In case the pressing force is about 1 Kg, a frictional force between the charging tube 2c and the foam layer 2b becomes large so that the charging tube 2c can not be pushed back by the bearing member. Hence, the force for pressing the bearing member becomes large more and more. Eventually, the edge of the charging tube 2c is turned over or deformed. Due to this, it becomes impossible to perform uniform charging. Such a phenomenon tends to occur as the frictional force between the charging tube 2c and the foam layer 2b is larger, and as the charging tube 2c is thinner (for example, 50-70 xcexcm), and as the material used for the charging tube 2c is softer.
It has been found that the embodiment 2 shown in FIG. 10 has the following problems. The first problem relates to a shift of the elastic cylinder 2d during the rotation. Since the elastic cylinder 2d loosely encases the shaft 2a, the elastic cylinder 2d moves in the axial direction in accordance with the rotation of the photosensitive drum 1 similarly to the case of FIG. 9. As a result, the elastic cylinder 2d is pressed against the bearing member together with the charging tube 2c. Upon this, a sound by friction is likely to be generated. Meanwhile, the elastic cylinder 2d is much thicker than the charging tube 2c and is hardly damaged. However, when the elastic cylinder 2d is strongly pressed against the bearing member, the edge of the elastic cylinder 2d is compressed, and creases are sometimes generated at that portion of the charging tube 2c. If such a phenomenon occurs, it becomes impossible to perform uniform charging. The second problem relates to non-uniformity of the conductive elastic cylinder. As to the elastic cylinder 2d formed by the extruding machine, a thicker portion and a thinner portion exist. Since the thickness of the charging tube 2c is thin, the outer diameter of the charging tube 2c has a larger portion and a smaller portion corresponding to the outer diameter of the elastic cylinder 2d when the charging tube 2c is integrally provided on the outer periphery of the elastic cylinder 2d. In case the outer diameter is larger, a pressure nip width is longer. By contrast, in case the outer diameter is smaller, the nip width is shorter. When the nip width is different, the amount of charge deposited on the surface of the photosensitive drum 1 is different. If portions having different outer diameters exist in one charging roller, it becomes impossible to perform uniform charging. In view of this, after the conductive elastic cylinder 2d is formed by the extruding machine, the outer surface is usually ground to obtain a predetermined constant diameter. Thus, the cost for a grinding process is necessary. The third problem relates to a problem in that items of reusable parts are less. When the charging tube 2c is soiled or is damaged and the charging roller becomes unusable, the shaft can be reused. However, the conductive elastic cylinder and the charging tube can not be reused so that they are scrapped.
In view of the foregoing, it is a primary object of the present invention to provide a charging unit which is hardly damaged at the edge of a seamless tube and can perform uniform charging.
It is a second object of the present invention to provide a charging unit which hardly generates creases on the seamless charging tube and can perform uniform charging.
It is a third object of the present invention to provide a charging unit which uses a conductive elastic cylinder formed by extruding machine, the surface thereof being not ground.
It is a fourth object of the present invention to provide a method for manufacturing a charging unit in which it is unnecessary to integrally form a conductive elastic cylinder on a shaft.
It is a fifth object of the present invention to provide a method for manufacturing a charging unit in which a lot of parts may be reused and is good for the environment.
It is a sixth object of the present invention to provide a method for manufacturing a charging unit in which a process for grinding a surface is unnecessary.
It is a seventh object of the present invention to provide a charging device and a transfer device using the charging unit according to the present invention.
The above and other objects are achieved by the charging unit constituted of a shaft, a conductive elastic cylinder loosely encasing the shaft, and a seamless charging tube loosely encasing the conductive elastic cylinder. An inner diameter of the conductive elastic cylinder is larger than an outer diameter of the shaft. An inner diameter of the seamless charging tube is larger than the outer diameter of the conductive elastic cylinder.
The seamless charging tube is made in such a manner, for example, that a semi-conductive polymer is formed in a tube shape by extruding machine. The semi-conductive polymer includes, for example, polyamide elastomer in which conductive carbon is mixed. The conductive elastic cylinder is made in such a manner that synthetic rubber in which conductive carbon and a foaming agent are added is formed in a pipe shape by extruding machine. The elastic cylinder and the charging tube loosely encase the shaft in order.
The charging device according to the present invention includes the above-mentioned charging unit, a bearing member and a press member. The bearing member holds both ends of the shaft of the charging unit. The press member presses the charging tube of the charging unit against a material to be charged by pushing the shaft toward the material to be charged. A charging voltage is applied between the charging tube and the material to be charged. The charging tube, the elastic cylinder and the shaft are driven to rotate in association with the movement of the material to be charged.
The transfer device according to the present invention includes the above-mentioned charging unit, a bearing member, and a press member. The bearing member holds both ends of the shaft. The press member presses the charging tube against the material having thereon a toner image by pushing the shaft toward the material having thereon a toner image. A transfer voltage is applied between the charging tube and the material having thereon a toner image with a recording medium disposed therebetween in order to transfer the toner image.
In a preferred embodiment of the present invention, a stopper is rotatably attached to the shaft to prevent the charging tube loosely encasing the elastic cylinder from continuing to move in an axial direction of the shaft during rotation of the charging unit. The stopper is disposed between the end of the charging tube and the nearest bearing member. Moreover, a regulating member may be attached for regulating the elastic cylinder so as not to protrude out of both ends of the charging tube. In order to prevent the stopper from dropping out of the shaft, a drop preventing member may be provided between the stopper and the nearest end of the shaft. This preventing member prevents the stopper from moving to the end of the shaft and dropping out from it. A stopper having a special structure may be used as the regulating member and the drop preventing member. Further, the bearing member of the charging device may be used as the stopper and/or the regulating member.
The charging unit according to the present invention has advantages relative to its performance, its cost, and the environment in recycling and discarding. Moreover, there is another advantage that the life of a photosensitive drum may be extended. These advantages are obtained by the structure in which the seamless charging tube and the conductive elastic cylinder are individually provided, and the charging tube and the elastic cylinder compensate aforesaid problems mutually. Further, the present invention solves the troubles regarding the end portion of the charging unit, and also solves the problems regarding materials of the respective member, shapes thereof, and a limit of the production accuracy of the conductive foam layer.
The life of the charging unit mainly depends on the life of the seamless charging tube. According on the present invention, when the seamless charging tube is damaged, only the seamless charging tube may be changed with a new one. Thus the shaft and the conductive elastic cylinder can be reused.